The present invention relates to a torque shaft assembly and, more particularly, to a torque shaft assembled from easily machined parts, with arms that are oriented one to another so that they move different stages of vanes in an optimal fashion.
The key to increased engine performance is increased engine overall pressure ratio. Engine overall pressure ratio is established in the compressor. The key to increased pressure ratio in an advanced compressor is to provide numerous stages of movable compressor vanes whose angles may be varied as the compressor is brought up to speed in order to prevent a condition called xe2x80x9cstallxe2x80x9d. Compressor xe2x80x9cstallxe2x80x9d is detrimental to the service life and condition of the compressor. Each stage of vanes may have optimal angle schedules that are different one from another, often in non-linear interrelationships. Furthermore, the vanes must replicate the desired angle schedule in both the opening and closing direction, which means that deflection of the moving system under actuation loads must be minimized.
Stage 2 is typically termed the master vane stage. The engine controller is typically programmed to monitor Stage 2, and move the torque shaft so as to move Stage 2 in accordance with a program keyed to engine speed during startup or to effect power turndowns at constant speed when required. A power turndown is used when the power demand on the grid drops such as during lunch time, for example, and the plant operator wishes to keep the turbine-generator set running and synchronized to the grid. The generators cannot be permitted to generate any more power than the amount of power being used by the grid.
When the vane schedule is changed, because of performance requirements or optimization tests, radical changes are usually made to the kinematics of the Variable Guide Vane linkage (VGV) system (referred to by some engineers as Variable Stator Vane (VSV) system). Usually, this means a new torque shaft. Typically torque shafts are cast and then machined to define the torque shaft arm clevis locations.
Certain new engines feature compressors with several stages of movable vanes that have non-linear schedules which necessitate out-of-plane torque shaft arm clevis locations. The out-of-plane torque shaft arm clevis locations make a cast one-piece torque shaft impractical. In addition, the axial spacing of the torque shaft arm clevis locations leave insufficient room to machine a cast torque shaft even if the torque shaft arm clevis locations are in-plane.
Furthermore, the Critical-to-Quality (CTQ) of the VGV system is vane angle accuracy. Accordingly, the present invention provides a torque shaft that addresses the problems associated with VGV systems, and particularly systems in which vanes have non-linear schedules and/or which otherwise preclude or inhibit the manufacture and use of cast, one-piece torque shafts.
The present invention provides a fabricated torque shaft that features a bolt-together design to allow vane schedule revisions with minimal hardware cost. The bolt-together design of the invention further facilitates on-site vane schedule revisions with parts that are comparatively small. The fabricated torque shaft of the invention also accommodates stage schedules that are different one from another in non-linear inter-relationships as well as nonlinear schedules for a particular stage of vanes. The invention also eases machining so as to prevent sag of a long shaft during machining.
Thus, the fabricated torque shaft of the invention is embodied in an assembly comprising a torque shaft main body having a forward end and a rearward end, bearings defined adjacent each end of the shaft main body, and a plurality of arm structures provided at spaced locations along said torque shaft main body for operatively coupling said torque shaft to a plurality of vane stages of a compressor; wherein at least one of the arm structures is detachedly secured to the torque shaft main body so that the at least one arm structure can be removed and replaced with another arm structure.